Geophysical Determination of Landfill Pollution Plume Extent at Olusosun Dumpsite, Lagos State, South West Nigeria

Olusosun landfill in Lagos State is one of the largest active dumpsites in West Africa, receiving over 3,000 tonnes of mixed municipal and industrial waste daily, and concerns about the extent of groundwater contamination from leachate migration have grown as urbanisation encroaches on adjacent residential areas. This study employs a multi-method geophysical approach combining 2D ERT, frequency-domain electromagnetic profiling, and vertical electrical sounding to determine the three-dimensional extent of the leachate pollution plume extending from Olusosun landfill. ERT profiles of 600 metres were acquired along 12 radial lines extending from the landfill perimeter in the principal groundwater flow direction. EM31 and EM34 profiling provided rapid preliminary reconnaissance of apparent conductivity variations beyond the ERT coverage area. VES stations at 35 locations beyond the ERT grid provided depth calibration for the electromagnetic conductivity maps. The combined dataset reveals a leachate plume characterised by resistivities of less than 5 ohm-metres extending 850 metres downgradient of the landfill boundary at depths of 0 to 20 metres. The lateral plume width at 500 metres from the landfill boundary is approximately 420 metres. A semi-confined sandy layer at 18 to 25 metres depth appears to provide lateral transport for deeper plume penetration. Comparison with groundwater quality data from 20 boreholes within the plume extent confirms elevated total dissolved solids, ammonia, and chloride concentrations consistent with the geophysical leachate signature. The study provides the most spatially comprehensive characterisation of the Olusosun leachate plume to date. Keywords: leachate plume, Olusosun landfill, ERT, electromagnetic, groundwater contamination